1. Field of the Invention
The present invention is in the field of night vision devices. More particularly, the present invention relates to a night vision device which uses an image intensifier tube to amplify light from a scene. This light may be too dim to be seen with natural human vision, or the scene may be illuminated substantially only by infrared light which is invisible to human vision. The image intensifier tube both amplifies the image from the scene and shifts the wavelength of the image into the portion of the spectrum which is visible to humans, thus to provide a visible image replicating the scene. Still more particularly, the present invention relates to such an image intensifier tube having a unitary ceramic body portion, as well as a photocathode and a microchannel plate spaced from one another to define a spacing dimension, this dimension being established by structure extending axially between the photocathode microchannel plate, and establishing this spacing dimension independently of tolerances and variability""s of the other components of the image intensifier tube. Methods of making of operating such an image intensifier tube are presented.
2. Related Technology
Even on a night which is too dark for natural human vision, invisible infrared light is richly provided in the near-infrared portion of the spectrum by the stars of the night sky. Human vision cannot utilize this infrared light from the stars because the infrared portion of the spectrum is invisible to humans. Under such conditions, a night vision device (NVD) of the light amplification type can provide a visible image replicating a night-time scene. Such NVD""s generally include an objective lens which focuses invisible infrared light from the night-time scene through the transparent light-receiving face of an image intensifier tube (I2T). At its opposite image-output face, the I2T provides a visible image, generally in yellow-green phosphorescent light. This image is then presented via an eyepiece lens to a user of the device.
A contemporary NVD will generally use an I2T with a photocathode (PC) behind the light-receiving face of the tube. The PC is responsive to photons of visible and infrared light to liberate photoelectrons. Because an image of a night-time scene is focused on the PC, photoelectrons are liberated from the PC in a pattern which replicates the scene. These photoelectrons are moved by a prevailing electrostatic field to a microchannel plate having a great multitude of microchannels, each of which is effectively a dynode. These microchannels have an interior surface at least in part defined by a material liberating secondary-emission electrons when photoelectrons collide with the interior surfaces of the microchannels. In other words, each time an electron (whether a photoelectron or a secondary-emission electron previously emitted by the microchannel plate) collides with this material at the interior surface of the microchannels, more than one electron (i.e., secondary-emission electrons) leaves the site of the collision. This process of secondary-electron emissions is not an absolute in each case, but is a statistical process having an average emissivity of greater than unity.
As a consequence, the photoelectrons entering the microchannels cause a geometric cascade of secondary-emission electrons moving along the microchannels, from one face of the microchannel plate to the other so that a spatial output pattern of electrons (which replicates the input pattern; but at an electron density which may be, for example, from one to several orders of magnitude higher) issues from the microchannel plate.
This pattern of electrons is moved from the microchannel plate to a phosphorescent screen electrode by another electrostatic field. When the electron shower from the microchannel plate impacts on and is absorbed by the phosphorescent screen electrode, visible-light phosphorescence occurs in a pattern which replicates the image. This visible-light image is passed out of the tube for viewing via a transparent image-output window.
The necessary electrostatic fields for operation of an I2T are provided by an electronic power supply. Usually a battery provides the electrical power to operate this electronic power supply so that many of the conventional NVD""s are portable.
However, the electrostatic fields maintained within a conventional image intensifier tube, which are effective to move electrons from the photocathode to the screen electrode, also are unavoidably effective to move any positive ions which exist within the image intensifier tube toward the photocathode. Because such positive ions may include the nucleus of gas atoms of considerable size (i.e., of hydrogen, oxygen, and nitrogen, for example, all of which are much more massive than an electron), these positive gas ions are able to impact upon and cause physical and chemical damage to the photocathode. An even greater population of gas atoms present within a conventional image intensifier tube may be electrically neutral but also may be effective to chemically combine with and poison the photocathode.
Conventional image intensifier tubes have an unfortunately high indigenous population of gas atoms within the tubexe2x80x94both those gas atoms which become positive ions and those much more populous atoms that remain electrically neutral but are possible of chemically reacting within the tube. Historically, this indigenous population of gas atoms resulted both in the impact of many positive ions on the photocathode, and in chemical attack of the photocathode. With many early-generation I2T""s, this resulted in a relatively short operating life.
As those ordinarily skilled in the pertinent arts will understand, later generation I2T""s of the proximity focus type have partially solved this ion-impact and chemical reaction problem by providing an ion barrier film on the inlet side of the MCP. This ion barrier film both blocks the positive ions and prevents them form damaging the PC, and inhibits the migration of chemically active atoms toward the PC. However, the ion barrier film on a MCP is itself the source of many disadvantages.
A recognized disadvantage of such an ion barrier film on an MCP is the resulting decrease in effective signal-to-noise ratio provided by the MCP between a PC of an I2T and the output screen electrode of the tube. That is, although the material of the ion barrier film itself acts as a secondary emitter of electrons, but only for those electrons of sufficient energy. Electrons of lower energy may be absorbed by the ion barrier film, so that this ion barrier film acts to prevent these low energy electrons from reaching the microchannels of the MCP. Secondary-emission electrons typically have a comparatively low energy. Recalling that about 50% of the electron input face of a MCP is open area, and about the same percentage is defined by the solid portion or web of the microchannel plates, it is easily appreciated that about half of the photoelectrons impact on the web of the MCP. Moreover, these photoelectrons which impact the web of the MCP result in the production of secondary emission electrons closely adjacent to the open areas of the MCP, and with low energies. These low-energy electrons lack the energy to either penetrate the ion barrier film, or to cause this film to liberate secondary electrons. So these low energy electrons are absorbed by the ion barrier film. The result is that in some cases, as much as 50% of the electrons that would otherwise contribute to the formation of an image by the I2T are blocked or absorbed by the ion barrier film and do not reach the microchannels to be amplified as described above. Thus, about the same percentage of the image information which theoretically could be provided by the tube is lost.
Another disadvantage of the ion barrier film is that it contributes to halo effect in the image provided by the conventional image intensifier tube. This halo effect may be visualized as photoelectrons incident on the web of the MCP, or on the ion barrier film itself, either themselves not penetrating this film to enter a microchannel and to be amplified, but bouncing off to again impact the film or the web at another location. At the other location, the process is repeated, with some of the electrons entering a microchannel, and some of the electrons again bouncing to yet a third location. This effect causes a halo or emission of light around locations of the image. This halo light emission does not correspond to a bright area of the scene being viewed. This halo effect reduces the quality of the image provided by an image intensifier tube, and reduces contrast values in this image.
Another problem with image intensifier tubes using an ion barrier film is the electron voltage that must be provided (i.e., by the use of a higher applied voltage between the PC and the MCP) to photoelectrons simply to compensate on a statistical basis for the electron barrier which is represented by the film itself. The ion barrier film itself requires about 600 to 700 volts of additional applied potential.
Yet another source of image halo in conventional MCP""s results from the excessive distance maintained between the PC and the front face of the MCP in these conventional I2 T""s. The conventional I2T""s generally have a gap from PC to MCP no less than about 250xcexc meter (+ or xe2x88x92 about 25xcexc meter). It is recognized that an important factor in the extent or degree of halo effect is the spacing between the PC and the MCP of an I2T. However, conventional I2T""s have not been able to provide a spacing as small at that achieved by the present invention.
U.S. Pat. Nos. 3,720,535, issued Mar. 13, 1973; 3,742,224, issued Jun. 26, 1973; and 3,777,201, issued Dec. 4, 1973 provide examples of microchannel plates or image intensifier tubes having an ion barrier film on a microchannel plate. Also, a construction of microchannel plate relevant to this present invention is taught in U.S. Pat. No. 5,493,111, owned by the assignee of this present application, and on which the inventor of this present application is also a joint inventor.
In view of the deficiencies of the conventional related technology, it is desirable and is an object of this invention to provide a night vision device which avoids or reduces the severity of one or more of these deficiencies.
Further, it is an object for this invention to provide an image intensifier tube which overcomes or reduces the severity of at least one deficiency of the conventional technology.
Thus, it is desirable and is an object for this invention to provide an improved I2T having a spacing between the PC and the MCP of the tube which is independent of tolerances or variability""s of the body of the tube.
More particularly, the present invention relates to an improved I2T having an improved housing with a portion formed of ceramic or other insulative material, and which portion provides for electrical contact with a MCP of the tube, and also allows the spacing of this MCP from the PC of the tube to be determined by a PC-to-MCP spacer(s) extending axially between the PC and MCP of the tube.
An additional object and advantage of this invention is the provision of an I2T having a high-voltage power supply in the form of an annulus which is axially aligned and stacked with the tube body (i.e., rather than in the form of an annulus surrounding the tube body), so that the envelope diameter of the tube is made smaller in comparison with conventional tubes.
Still further, an object for and advantage of this invention is the provision of an I2T having a tube body with no radially outwardly exposed or provided electrical contacts. In other words, the ceramic or other insulative body portion of the present tube body provides all electrical contacts for operation of the tube, and these are all axially aligned.
Accordingly, it is an object and advantage for this invention to provide an I2T with an axially-stacked high-voltage power supply which makes electrical connection to the tube via axially disposed contact pads of the tube body.
Further, it is an object for this invention to provide such an I2T having a MCP which is free of an ion barrier film, and thus provides an improved level of signal-to-noise in the tube.
It follows that an object for and an advantage of this invention is the provision of an I2T which has an extraordinarily low level of image halo.
To this end, the present invention according to one aspect provides a night vision device comprising an image intensifier tube having a body holding: a photocathode, a microchannel plate, and a display electrode, the image intensifier tube receiving low-level or long wavelength light and responsively providing a visible image, the image intensifier tube comprising: the body including a body ring-like portion defining a step upon which is disposed deformable electrical contact structure, this contact structure making electrical contact with the microchannel plate; and axially extending insulative spacing structure extending between the photocathode and the microchannel plate and physically touching at least one of the microchannel plate and photocathode to trap the microchannel plate in a selected axial position on the step and establish a selected fine-dimension spacing between the microchannel plate and an active portion of the photocathode, and the body further including a deformable and axially variable sealing portion sealingly uniting the body portion with a window member carrying the photocathode; whereby the axially variable sealing portion and deformable electrical contact structure cooperatively accommodate dimensional variability""s for both the body portion and the window member, and the spacing dimension is independent of these dimensional variabilities.
The Applicant has discovered that, in contrast to the conventional technology, and by use of the present invention the spacing between the PC and the MCP in an I2T may be reduced. This reduction of spacing dimension may be from about 50% of the conventional value to as much as essentially an order of magnitude less than the conventional and current spacing (i.e., to substantially about 25xcexc meter or less). Most preferably, the gap from PC to MCP may be reduced to as little as about 20xcexc meter. The image halo image effect of the present image tube is correspondingly reduced in comparison to conventional I2T""s.
Further, the I2T according to the present invention may operate on lower applied voltages between the PC and MCP, so that the applied electric field between the PC and MCP is maintained at about the same level as that employed in conventional I2T""s.
A further advantage results from the reduced electron energy necessary to introduce electrons into the microchannels of the MCP in comparison to conventional image intensifier tubes. Because the microchannels of an image intensifier tube embodying the present invention are open in the direction facing the photocathode (no ion barrier film is present to restrict electron entry) the photoelectrons have essentially no barrier to overcome. This is in contrast to conventional proximity focused image intensifier tubes, which have an ion barrier on the input side of the MCP. As explained above, in conventional I2T""s electrons must effectively penetrate the ion barrier to get into the microchannels of the conventional image intensifier tube. Thus, the voltage applied to the photocathode of an image tube operated according to the invention can be lowered, while still providing an adequate level of applied electric field, and while also still providing an adequate flow of photoelectrons to the microchannel plate. This advantage allows use of a smaller and lower-voltage power supply.
Still further, serial manufacturing of image intensifier tubes embodying the present invention is made considerably easier and less expensive because the fine-dimension spacing of the photocathode from the microchannel plate is independent of dimensional variabilities of the window member and of the tube housing. In other words, while conventional image intensifier tubes depend upon control of tolerance stack-up dimensions for the components of the tube body in order to control the PC-to-MCP gap, the present invention allows a deformable structure to variably yield during manufacturing of the image intensifier tube, and by so yielding to compensate for tolerances of both the window member and of the tube body. The result is both a new freedom from the necessity to control dimensional tolerances of the window member and tube body to high standards, and a heretofore unobtainable precision and repeatability in establishing the fine-dimension PC-to-MCP gap.
These and additional objects and advantages of the present invention will be apparent from a reading of the following detailed description of preferred exemplary embodiments of the invention, taken in conjunction with the following drawing Figures, in which the same reference numbers refer to the same feature, or to features which are analogous in structure or function.